This invention relates generally to scanning electron beam scanners and more particularly to a method and apparatus to produce and maintain electron beam space-charge neutralization.
Scanning electron beam scanners, such as those used in computed tomography (CT) imaging systems, produce cross-sectional and three-dimensional (3D) images of the human body and objects such as baggage and cargo. The scanning electron beam permits fast scan speeds, e.g. 50 msec/scan, so that rapidly moving objects such as the human heart can be imaged.
Conventional scanning electron beam scanners include an electron gun that produces an electron beam inside a highly evacuated chamber. The electron beam expands from its point of origin, e.g. the electron gun, because of the mutual electrostatic repulsion of the electrons in the beam. Where the beam is sufficiently large, it passes through a magnetic lens and a dipole deflecting magnet which scans the electron beam along a target located at the far end of the vacuum chamber to produce x-rays. These x-rays pass through the object being imaged and their intensity is measured by an array of x-ray detectors. X-ray absorption along lines through the object can be calculated and a cross-sectional image of the object can be digitally reconstructed.
For medical applications in particular, the electron beam is required to have an energy of up to approximately 140 keV, a current of up to approximately 1 Amp, and a beam spot width of less than 1 mm. Such parameters are mutually incompatible for an electron beam in pure vacuum where the space-charge repulsion of the beam produces much larger beam spots. More specifically, when the electron gun is activated the electron beam is produced. Initially, the electrons located near a center of the electron beam repel the electrons located at a radially outer edge of the electron beam causing the diameter of the electron beam to expand making focusing the beam more difficult. To overcome this problem, one conventional scanning electron beam scanner neutralizes the space charge of the electron beam by introducing nitrogen into the beam tube and maintaining the pressure within the beam tube at approximately 10−6 Torr. This pressure, which is much higher than a normal good vacuum pressure, ensures that the electron beam will neutralize itself rapidly by means of its own beam-generated plasma. More specifically, the negatively charged electrons in the beam interact with the gas creating positive ions. As the ions buildup in the beam, the width of the focal spot of the beam decreases to a small value limited only by the emittance of the beam.
To maintain approximately 10−6 Torr vacuum pressure within the beam tube, the conventional scanning electron beam scanner includes a combination of cryopumps and pressure control servo-valves. However, the cryopumps and servo-valves are relatively expensive and require a sophisticated mechanical vacuum system, thus increasing the overall cost and complexity of the conventional scanning electron beam scanner.
Moreover, during a conventional scanning procedure, the electron beam is incident on a conventional target where it produces x-rays. After the scanning procedure is completed, the electron gun that produces the electron beam is deactivated. However, when the conventional electron gun is deactivated all the positive ions accumulated in the electron beam are lost. The ions must then be regenerated each time the electron gun is reactivated and the electron beam enters the scan chamber to again neutralize the space charge of the electron beam. The time required to regenerate the quantity of ions necessary to neutralize the space charge of the electron beam each time the electron gun is activated may not be a significant issue in some applications. However, in other applications such as a cardiac scanner or a baggage handling system, the time required to re-neutralize the space charge of the electron beam may cause significant time delays that affect the overall performance of the system unless the vacuum system is operated at as high a pressure as possible.